1. Field
The present specification generally relates to optical fibers and, more specifically, to few-moded optical fibers with low loss and small differential group delays.
2. Technical Background
Few-moded fibers (10 or fewer LP modes at 1550 nm) have been designed to transmit optical signals in two or more LP modes. Spatial mode converters can be used to multiplex the optical signals into the fiber and the signals can be decomposed at the receiver using electronic dispersion compensation. The few-moded fibers previously proposed for WDM communications systems have step index cores in which the diameter is increased relative to the single mode fibers in order to support additional modes. One problem with these designs is that there are large delay differences between the fundamental mode and the higher order modes (HOMs). If there is even a small amount of mode mixing in the fiber, the pulses arriving at the detector are degraded due to multipath interference, and this can lead to unsurmountable bit error rate penalties. Another problem is that increasing the core diameter leads to high microbending losses, and although the higher order modes (HOMs) can be designed to have theoretical cutoffs above the 1550 window, these modes are leaky and are not suitable for long transmission spans. A third problem is that the large effective area of the step index fiber is achieved by reducing the refractive index of the core, and this reduces the numerical aperture (NA) of the fiber. This smaller NA can increase both the coupling losses and alignment sensitivity between the spatial mode converter or optical transceiver and the fiber.
Accordingly, a need exists for alternative designs for few-moded optical fibers with low loss, small differential group delays (DGD) and large numerical aperture.